As above really, I have sanded them back to base and now need to respay them, do I have to prime them and if so can I use a plastic primer?

ta

No mate.. for fibreglass, never..

Try Paints 4u, they sell primers for fibreglass.

Cheers

i used to be a fibre glasser (laminator) be carefull when sanding you may get pitting if you go throught the gel coat and this is a nightmare hen you come to painting you wil end up with dimples any good primer will work and i would sudgest using some as primer is pretty thick and will fill in any pitting that may happen treat grp like any other thing you will paint and you will get good results

Fibreglass is very easy to paint, but bodyshop has to know how to do it.. You have to use only good quality materials:
1. putty spray
2. primer (not plastic primer)
3. two-component varnish

Also better use some sprayfiller and sand them with a 20-30cm block. They are not as smooth as oe plastic ones.
Here are mine after one go: http://sfk.ibk.se/forum/attachment.php?s=eb80ec8f13b7921cdfbc005d65179090&postid=100901

Plain to see!